1. Field of the Invention
The present invention relates to a smoke detecting apparatus and, more particularly, to a smoke detecting apparatus capable of detecting both smoke and fine particles such as dust.
2. Description of the Related Art
Hitherto, as a smoke detecting apparatus for detecting smoke caused by a fire, and a circuit therefor, a photoelectric analog smoke sensor disclosed in Japanese Patent Laid-Open No. 63-32690, and a smoke detector and a photoelectric smoke detecting circuit disclosed in U.S. Pat. Nos. 4,166,960 and 4,654,644, have been known.
In this photoelectric analog smoke sensor, a light emitting chamber and a light receiving chamber are disposed in a chamber which is formed into a labyrinth. The light receiving chamber is placed at a position where light emission from the light emitting chamber is not directly received, so that diffused light caused by smoke entering the chamber is detected by the light receiving chamber, and a signal corresponding to the smoke density is obtained on the basis of the amount of received light in the light receiving chamber.
In the photoelectric analog smoke sensor, a light-emission drive circuit for making light emitting elements such as LEDs emit light intermittently is disposed in the light emitting chamber, and a light-receiving signal amplifying circuit provided with a light receiving element, such as a photodiode, is disposed in the light receiving chamber.
When diffused light caused by smoke in the chamber is detected by a light receiving element, a signal at a level corresponding to the smoke density is photoelectrically converted by the photodiode in the above-mentioned light-receiving signal amplifying circuit and then amplified. The output from this light-receiving signal amplifying circuit is integrated by an integrating circuit and then amplified by a DC amplifying circuit. In this way, a conventional smoke detector obtains an analog signal having output characteristics required by an automatic fire notification system.
However, in such a conventional photoelectric analog smoke sensor, the integrated amount of diffused light is detected. Therefore, the integrated amount is small in an area where the volume of fine particles caused by a fire is small. As a result, it is not possible to detect very small amount of smoke generated in the initial period of a fire.
On the other hand, since fine particles such as dust cannot be detected by the conventional photoelectric analog smoke sensor, it is not possible to distinguish dust and water vapor from smoke, nor to distinguish environmental abnormalities such as the contamination of the inside of the chamber at the same time smoke is detected. Hitherto, examples for detecting fine particles are an indoor environment monitoring system disclosed in Japanese Patent Laid-Open No. 2-254340; a fine particles sensor disclosed in U.S. Pat. No. 4,226,533; a sampling apparatus for analyzing gases contaminated with much dust disclosed in U.S. Pat. No. 4,459,025, and the like. Another example for preventing erroneous notification caused by fine particles such as dust, is a particle-size measuring type smoke detector disclosed in Japanese Patent Laid-Open No. 2-300647.
Accordingly, in particularly a clean room, a fine particles detecting sensor is disposed to monitor dust first. Along with this sensor, a detector such as the above-described photoelectric analog smoke sensor is disposed to prevent an accident due to fire. In this case, the cost of the fine particles detecting sensor is high. Therefore, there has been a demand to develop an apparatus capable of detecting environmental abnormalities at low cost. The application of the particle-size measuring type smoke detector may be considered. However, this detector is incapable of detecting fine particles of 1 .mu.m or less, and the detector is very expensive.